390 4. ALLOXAN 



nicotinamide might protect directly by binding at the NAD sites, or (4) the 

 excess NADH formed from nicotinamide might reduce the alloxan to dial- 

 urate. Younathan (1962) showed that the microsomal fraction of kidney 

 and liver homogenates is necessary for the inhibitory action of alloxan on 

 cycle oxidations, and postulated that alloxan might be converted to some 

 amobarbital-hke substance capable of inhibiting NAD-dependent oxidations. 

 Thus nicotinamide could protect either by increasing the NAD levels in 

 the /?-cells or by providing excess NAD to replace that lost from hyper- 

 permeable mitochondria. The interest in the protective action of barbi- 

 turate is that the structure is similar to alloxan. Barbiturate is not diabet- 

 ogenic but it may bind to the sites with which alloxan reacts and so pro- 

 tect the /?-cells (Martinez, 1955). If this is so, it indicates that alloxan does 

 not react rather nonspecifically with membrane or enzyme SH groups, 

 but attacks some site with a complementary structure. 



The protection exerted by glucose is especially interesting in that it 

 might provide evidence for the mechanism by which alloxan modifies 

 /?-cells. Kass and Waisbren (1945) noted that feeding or injecting glucose 

 6 hr before the administration of alloxan reduces the susceptibility of 

 previously starved rats to alloxan; however, no effect was seen when the 

 glucose was injected 1 hr before the alloxan, which makes it likely that the 

 protection observed was of a different kind than studied subsequently. It 

 remained for Bhattacharya (1953, 1954) to demonstrate that glucose 

 protects when injected immediately before alloxan, but not when given 

 afterward. There is no evidence for reaction between glucose and alloxan. 

 Mannose is about one half as effective as glucose, and fructose is about 

 one seventh as effective. A competitive-like relationship was established 

 and it w^as thought that alloxan may inhibit hexokinase, and hence glucose 

 uptake and utilization, this enzyme being protected by the administered 

 glucose. Arteta et al. (1954) confirmed this protective action in dogs and 

 suggested that the concentration of glucose around the /?-cells may control 

 the sensitivity to alloxan. 



The following theories for the protection have been advanced: (1) the 

 direct protection of hexokinase mentioned above, (2) the metabolism of 

 glucose provides something, perhaps high-energy substances, which are 

 protective (Villar-Palasi et al., 1957), and (3) the enhancement of the re- 

 lease of epinephrine by glucose (Vollmer et al., 1960). Despite the fact that 

 Villar-Palasi et al. (1957) found hexokinase from dog pancreas to be fairly 

 readily and rapidly inhibited by alloxan, and glucose to exert some pro- 

 tection against alloxan, they concluded that this is not the mechanism, 

 mainly because the ratio of the protective abilities of glucose, mannose, 

 and fructose does not correspond to the affinities of these sugars for kexo- 

 kinase (it might be noted that in such discussion there is often some con- 

 fusion between affinities and phosphorylation rates of the various sugars). 



